Download Nutritional considerations in attempts to change the chemical

Vol.
29
Plant breeding and nutrition
3
protein wheat and barley and in the possibilities of manipulating amino acid composition have encouraged breeders to proceed more positively with biochemical
problems. I n many ways this resurgence has been due to the wide publicity given
to the potential of the opaque-z gene in maize, and the high-lysine story has
stimulated expectancies which I consider are rather rash in some cases.
These developments have provided, we hope, a new dimension to crop breeding,
and have brought together biochemists, nutritionists, geneticists and plant breeders
in intimate association. New genes or genotypic associations are being feverishly
sought after : world collections are being ransacked, and recourse to the exploitation
of the newly available cytogenetic and breeding techniques has already started.
We shall hear more of these matters in the papers to follow.
Nutritional considerations in attempts to change the chemical
composition of crops
By K. J. CARPENTER,
Department of Agricultural Science and Applied Biology,
University of Cambridge
Improving the composition of crops by breeding has obvious advantages over
many other ideas for raising nutritional standards in the world. Because of the
long-term nature of such projects it is especially important that objectives should be
thoroughly discussed before the criteria and priorities of selection are decided on.
History shows that effort can be wasted as a result of wrong assumptions about
the nutritional aspects of a problem. For example, in the 1930s it was recognized
that pellagra, a serious disease in countries where the poor lived on maize as their
staple food, was a deficiency disease responding to the vitamin nicotinic acid. I t then
seemed obvious that breeders should try to develop strains of maize having a higher
content of nicotinic acid. And yet the idea was unsound. It now seems that the
nicotinic acid is present in all grains in a nutritionally unavailable form (Chaudhuri
& Kodicek, 1960). Other grains are better than maize in preventing pellagra because
they contain more of the amino acid tryptophan. I n the body, tryptophan (in excess
of that required for protein synthesis) can be metabolized to nicotinic acid (Krehl,
1949).As would be expected, opaque-2 maize, with its higher tryptophan content,
does prevent nicotinic acid deficiency in experimental animals (Bressani, Elias &
Gomez-Brenes, 1969).
This example illustrates the responsibility of nutritionists for checking as far as
possible that the criteria they are suggesting for improving crops are effective ones.
There seems a natural tendency for the nutrients which are newly discovered or
the object of recent research to receive an undue amount of attention. Plant
breeders must make definite ‘yes or no’ decisions about their priorities. If
the nutritionists will not recommend nutritional priorities the selection will have
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to be done by non-specialists from their general impression of the current state of
nutritional opinion. Table I represents an attempt to summarize such impressions
at three periods of time.
Table
I.
An impression of opinions ahout desirable changes in human diet at dijferent
periods
Future
1930-50
1950-65
Vitamins
Animal fat
Lysine
Protein
Vegetable fat
Vitamins
Minerals
? Starch
.hima1 fat
? Sucrose
T o be increased :
Highest priority
Second priority
Y o problems :
Excessive already:
Proteins
Minerals
Starch
? hlcthioninc
? Trace minerals
3
Sophisticated diets .for animals
Crops may be used either directly as human food or indirectly as fodder for
productive animals. I will start by considering diets for intensively housed pigs and
poultry in this country. We have a free market economy with complete flexibility
of formulation, provided that nutrient requirements are met, with none of the
problems of human dietetics as regards what changes are socially acceptable.
Normally there is no expectation of making better diets, since they are thought to
be fully balanced already; the whole operation of the compounding industry is to
make equally good diets more economically and their success can be checked by
actual performance.
It is difficult to split up the cost of providing the different nutrients in such
mixtures when the ingredients are themselves complex materials, but it may be
roughly estimated that
of the cost is for the provision of energy, 30% for
protein and 5% for vitamins and minerals, with a final sok for compounding the
ingredients together. If a single crop could be produced that was completely balanced
in itself, there mould be substantial advantages because mixing costs would be
eliminated. Otherwise, there is little economic advantage for the sophisticated market
in selecting crops for higher concentration of vitamins or minerals that can be added
very cheaply from other sources during ordinary mixing. Major economies in the
cost of balanced rations can only come from cheapening the supply of energy or
protein, or both.
Presumably there will continue to be the greatest interest in the improvement
of those crops that are already good yielders of energy and protein. T h e major leastcost energy sources for dry, concentrated diets in this country are the cereal grains.
Although the cereals contribute very important amounts of protein, even a 40%
increase above present levels would be insufficient to balance diets for poultry and
young pigs. There is therefore an interest in improved sources of home-grown
supplementary protein and the legumes form the second main subject of the present
symposium.
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Energy Lvalues’for intensive diets. T h e energy values of materials used for pig and
poultry diets are commonly expressed in units of ‘metabolizable energy’ (ME),
i.e. the gross energy of the food (measured as heat of combustion in a bomb calorimeter) less the gross energy of the fraction not combustible by the animals themselves (measured by bomb calorimetry of faeces and urine). There is reasonable
evidence that the same quantity of M E from different cereals is of the same net energy
value for the needs of these stock (cf. Carpenter, 1962).
T h e dry matter of one grain can have a slightly higher gross energy than that of
another if it has a lower mineral content or a higher oil content or both. But there is
little scope for lowering the former, and higher oil levels could be disadvantageous
in other ways. T h e second and more important variable affecting ME is the degree
of digestibility of the organic matter of the grain; this can vary greatly amongst the
individual carbohydrates that are found in grains. T h e overall effect is illustrated
in the following representative values (Hill, Anderson, Renner & Carew, 1960;
Potter & Matterson, 1960):
Gross energy
(kcal/s)
Maize
Sorghum
Wheat
Barley
Metabolizable
energy (chicks)
(kcalis)
4.06
3’50
4.11
3’3 I
3.10
4’04
4.02
2.64
ME as
O0
of gross mergy
86
81
77
66
Protein quuntity and quality. When a figure is stated for the ‘protein content’ of a
crop, it is usually the ‘nitrogen content x 6-25,. The digestibility is normally high
(90-95%) for concentrate foods and it is the variation in amino acid composition
that is important. Since at least nine of the twenty or so amino acids that go to
make up proteins have to be provided in the diet, there is not one requirement for
protein but at least nine separate requirements for individual amino acids.
I n practice, some of the essential amino acids are always present in the proteins
of a mixed diet in lower proportions than they are in the mixed tissue proteins of
the animal eating them. When the lysine concentration of the dietary proteins is
half that in the body proteins it follows, since lysine cannot be synthesized in the
body, that from IOO g of such dietary protein one could only expect a synthesis of
50 g body protein, at maximum. If this rate of conversion were confirmed in animal
experiments, the protein would be said to have shown a ‘net protein utilization’
(NPU) O f 50?/0.
If a complete chemical analysis for the amino acid composition of the dietary
protein has been carried out, the level of each amino acid can be compared with that
in a perfect or ‘reference’ protein for the animal concerned. If it is found that lysine
is present in lowest proportion to requirement (i.e. soy)), then it is said that the
chemical score of the dietary protein as a whole is 50%. Comparison of published
‘chemical scores’ and NPU values indicates that most of the variability in nutritional
value of the proteins in unprocessed foods can be explained by the extent of their
deficiency in the first limiting amino acid (Block & Mitchell, 1946-7; McLaughlan,
Rogers, Chapman & Campbell, 1959).
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Table 2. Representative values for the crude and net protein content of important foods
(based on tables of Kuppuswamy, Srinivasan @ Subrahmanyan, 1958)
Food
Wheat
Barley
Sorghum
Maize
(opaque-;? maize)
Crude protein
content (%)
S e t protein utilization (o/)
(and first limiting amino acid)
I2
7‘2
10
6.0
9
8.5
11.5
50 J
? 75 ( ? isoleucine)
Phaseolus beans
25
Peas
Soya beans
Groundnuts
23
35
28
55
25t
75t
75 t
threonine)
80 (methionine)
7j (methionine)
95 -
Cow’s milk
Beef muscle
Whole egg
Net protein
content (%)
4’5
4’3
? 8.6
I1
45”] (methionine)
&,
(methionine +lysine+
I0
23
15
*Values obtained only after cooking.
+On a dry-matter basis.
It has been a common convention to multiply the actual protein content of a food
by its percentage NPU (or an estimate of NPU calculated from chemical score) to give
a figure for ‘net protein value’ or ‘complete protein’ content of a food (Howard,
Monson, Bauer & Block, 1958). This has been done for a range of foods in Table 2 .
T h e ‘protein’ (i.e. the composite mixture of proteins present) in each of the
cereal grains is limiting in lysine. This seems to point strongly to the value of
O
L1oo:o
50:50
0:lOO
Protein proportions
Fig. I . The calculated chemical score of whole wheat and Phaseolus bean proteins in differingproportions.
The shaded area on the left represents the improvement possible from added lysine and that on the right
the improvement from added methionine. (The reference standard is taken from Bender (1965) with
additional analyses from Kakade & Evans (1963)).
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Plant breeding and nutrition
supplementing cereals with lysine, either directly as the synthetic supplement or,
in the long term, by breeding varieties of higher lysine content. However, most of
the protein sources such as legumes, that are used to supplement the cereal proteins
in balanced diets, have a higher lysine content and are limited by their relative
deficiency in the sulphur-containing amino acids, i.e. methionine and cystine.
Which amino acid one would expect to be limiting in a cereal-legume mixture
depends on the proportions, as is illustrated in Fig. I. T h e reference standard was
based on rat experiments. Slightly different standards are used for other species
but the figure illustrates some general points: (a) raising the level of any amino
acid but the one limiting the actual mixture is of no value; (b) increasing the level of
the first limiting amino acid can only raise the value of the mixture to the level
limited by the next amino acid; and (c) mixing two proteins may give mutual
supplementation without the addition of amino acids. Patwardhan (1961) has
confirmed the supplementation between cereals and legumes in feeding trials
with rats.
Economic value of dzfferent changes. We have now considered several nutrientscalories, protein, lysine and methionine. The problem is to know their relative
economic importance for breeding projects. Prices fluctuate with time and place
but it is still instructive to consider even a single situation. We have chosen barley
and made calculations, following commercial practice, with feeding-stuffs at prices
expected to prevail in the autumn of 1969 in southern England, including DLmethionine at E45o/ton and L-lysine hydrochloride at E765/ton ( D. R. P. Cressall
& K. J. Carpenter, unpublished results).
An analogue computer was instructed to choose least-cost mixtures that met the
nutritional criteria specified for each particular class of livestock. Beginning with
young broilers, the price of barley was then varied until the price was found that
just corresponded to an inclusion rate of ao%-in this instance AIS.I/ton. Standard
barley (i.e. with crude protein IO%, lysine 0*35%, methionine 0-150/~,cystine
0.2376, ME 2.64 kcal!g, etc.) was then replaced by a hypothetical barley of 10%
higher ME value; the price at which it was included at 20% in the new least-cost
formula was A21-7/ton, i.e. rzoo4 of the corresponding price for the standard barley.
These calculations were continued with the results set out in Table 3.
Table 3. Effect of changes in composition on the calculated 'opportunity prices' of
barley and$eld beans in least-cost diets of dzzeerent types
Barley
___h
f-
Type of diet
1,evel of inclusion of test material (7;j
Opportunity price of standard crop (Liton)
Relative opportunity prices for modified crop:
( I ) ME up 1096
(2) Lysine up 2096
(3) Methioninetcystine up 20%
(4) Protein up 2 0 % (amino acids in
proportion)
Broiler
starter
Battery
layer
7
20
20
Heavy
hog
50
18.1
17.2
23.2
Field
beans
Pig
rearer
Pig
rearer
22'0
5
36.1
IOj
50
I20
I20
108
I 06
104
I00
I00
I01
103
I00
104
105
I00
I00
109
II 0
'04
104
II0
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For the pig diets a similar procedure was followed, with the standard level of
inclusion set at 50%. A second set of figures for a pig rearer diet was based on the
situation of a pig feeder growing his own barley and using it as the sole cereal in his
rations-it was then calculated how much more he could value his barley per ton if
he grew a modified strain and his feeding was still to cost the same. T h e results were
almost exactly the same as those obtained by the first method.
T h e most striking figures in Table 3 are of a 10% increase in ME giving an
apparent 20y0 increase in economic value of barley when used in broiler and layer
rations. This arises from the common specification for such poultry diets that they
must contain not less than 2-65 and 2.80 kcal/g respectively. Since the formulas
have to include mineral supplements with no energy value, the remaining materials
must have a higher average ME. Standard barley is penalized because it can only
be included if ‘paired’ with another material of higher ME, and the concentrated
sources of energy, such as maize and fat, are usually expensive. I n pig rations less
concentrated mixtures are acceptable or even preferred :standard barley is more highly
valued there and the premium for a 1076 energy increase is only 6 4 % (Table 3).
T h e economic effects of changes in the protein of barley are what might be
expected, with lysine being a cost-limiting factor in three of the diets and the sulphurcontaining amino acids taking over this position in the layers ration. This is in
general agreement with American experience with modified maize, though Cromwell,
Kogler, Featherston & Cline (1968) found that even with growing chicks increasing
the lysine content gave no benefit while increasing methionine lcvel did have an
effect. T h e values of barleys for pig rations are generally similar to the results of
earlier calculations (Carpenter & Taylor, 1968). I t is clear that both increases in
total protein content (of the same amino acid composition) and increases in the
proportion of lysine in the grain increase its value. Hadley (1966) in a study of the
economic value of opaque-z maize in pig feeding has estimated that it would be
worth 7-22’$/,more than ordinary maize, according to the relative prices of maize
and soya-bean meal.
Such information allows comparison of the value of changes in cornposition
with the extra returns that could come from varieties giving higher yields of the
same composition. T h e further problem of how a market can be organized so that
growers receive appropriate recompense for crops of improved composition will be
considered later (Mossberg, 1970).
Experience from feeding trials. There is significant variability in the ME of different
samples of barley for chicks (Lcong, Jensen & McGinnis, 1962). This has not yet
been correlated for certain with simple analytical characteristics, although some
suggestions have been made (Burnett, 1966). Each sample, therefore, requires a
small feeding test and approximately I kg is needed. Some samples of low ME
may be improved by preliminary soaking or enzyme treatment, or both, that appears
to modify some of the structural carbohydrates which are present in variable
amounts and are responsible for reducing general digestibility (Leong et al. 1962;
Potter, Stutz & Matterson, 1965). This aspect seems worth systematic investigation
in connexion with breeding programmes.
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Other speakers will be describing the progress being made in changing the
protein and amino acid contents of grains. With opaque-2 maize there is no doubt
that the large changes in chemical composition have been accompanied by corresponding changes in nutritional value (Cromwell, Pickett & Beeson, 1967; Cromwell
et al. 1968), i.e. the extra nutrients found on analysis are biologically available.
With the smaller changes that have been found so far in other grains it is, of course,
more difficult to confirm the effects by actual feeding trials. Jones, Cadenhead &
Livingstone (1968), comparing two samples of barley in a trial with bacon pigs,
obtained a just significant advantage with the sample containing more lysine. On
the other hand, Brune, Thier & Borchert (1968) could find no correlation between
the feeding value of a series of barley samples (differing in fertilizer treatment as
well as variety) and their amino acid composition as determined by chemical analysis.
It may be that differences in digestibility, of the kind already considered in relation
to energy values, are operating to produce such results. Alternatively the inherent
variability between animals may be obscuring genuine, but small, differences
between the test materials.
Human diets
Vitamins. Man requires the same range of nutrients as do the single-stomached
domestic animals, with vitamin C in addition. There are still some areas of vitamin
deficiency-particularly deficiency of thiamine, amongst those with refined rice as
their staple, and of vitamin A. Bulk supplies of synthetic vitamin A are now so
cheap that their cost is not the limiting factor, but it seems an insoluble problem to
fortify a child’s diet if the mother lives in a remote village, buys no processed food
and is not in contact with any Government service. Under such conditions the
introduction into the area of a new strain of the traditional staple crop with a higher
vitamin A content could make all the difference. Presumably this could only be
achieved with crops such as maize and sorghum where carotenoids are already
present in some strains.
It has been past experience that nutritionally improved crops will only be adopted
if they have all the accustomed culinary qualities (e.g. texture and flavour) as well
as giving good yields. Even the colour of extra carotenoids may make an ‘improved’
variety unacceptable. With the B vitamins, and particularly thiamine, it is important
to consider only the concentration in the cereal as eaten. Extra vitamin present in
the portion of the grain that is lost during preparation of a food obviously has no
significance.
Calories and protein. For the poor of many developing countries there is a scarcity
of calories, so that they can maintain their energy balance only by curtailing their
physical activity, even when they spend so much of their income on staple foods
that little is left for other things.
Human growth and reproduction rates are, in proportion to size, very much less
than those in animals. As one would expect, therefore, man’s requirements for
protein in relation to calories are also considerably lower (Fig. 2 ) . For example,
in the studies of Widdowson & McCance (1954.) schoolchildren grew normally on
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diets in which only 12% of the calories came from protein (87%) of this being
vegetable protein, mostly from white bread) and which contained no supplementary
vitamins or amino acids. Nevertheless, there is a fairly widespread protein deficiencv
disease (kwashiorkor) amongst newly weaned, preschool children in some tropical
countries.
c
Defic
Human
Pigs
Chicks
Fig. z. A diagram representing a scale of P (76 of dietary calories coming from protein) with thc
position of individual foods along the scale indicated (after Autret & Jacquot, 1961). The maximum
amount of protein required by growing children receiving a mixed diet is represented as a fulcrum
on which the scale is balanced. The range of values for man, and approximate balance points for
productive pigs and poultry, are also indicated. The values for man are based on F A 0 (1965) estimates
with an allowance for the dietary protein having a score of 60.
Lysine or methionine. Whether or not lysine is in general the first limiting factor in
human diets where protein nutrition is suboptimal is a matter of controversy. T h e
subject has come to a head now that it is technically possible to produce synthetic
lysine on a very large scale. Some are in favour of using it, for example, as a n additive in all wheat shipments sent to India (Howe, Jansen & Gilfillan, 1965; Howe,
Jansen & Anson, 1968); others are sceptical (Hegsted, 1968; King, 1968; Gounelle,
'969).
Although lysine is certainly the first limiting amino acid in cereals taken alone,
it has been found that for all examples of average national diets, even from countries
where cases of protein deficiency are seen amongst the children, the first limiting
amino acid is methionine (Miller & Donoso, 1963). This was so both by calculation
of the chemical composition of the mixed protein of the diet and by actual feeding
trial of similar mixtures with rats. More recent calculations of the composition of
national average diets, based on F A 0 information, confirms that lysine hardly
ever appears to be the limiting amino acid with characteristic regional diets, and
that it is usually the sulphur-containing amino acids (Autret, Pirissit, Sizaret &
Cresta, 1968).
Those stressing the importance of lysine would probably argue that where the
staple is a cereal we are concerned not with the average member of a community
but with the small minority who become ill with protein deficiency because they
receive diets of less-than-average quality. These presumably have their lysinedeficient staples supplemented with a greater proportion of 'empty' calories (i.e.
fats, starches and sugar) and less of the protein-rich supplements, such as animal
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products and legumes, that are rich in Iysine. Experimental results from India
indicate that a children’s orphanage diet, based on Kaffir corn (Sorghum vulgare),
does give better growth when supplemented with lysine (Doraiswamy, Subramanya
Raj Uns, Venkat Rao, Swaminathan & Parpia, 1968). This controversy is, of course,
perplexing for breeders trying to decide whether to select for lysine specifically or
for greater protein in cereals.
I n communities where the staples are not cereals but starchy materials such as
cassava or plantains with an extremely low protein content there is, necessarily,
an absolute reliance on the supplementary foods to supply an adequate balance of
protein in the diet. T h e contribution of legumes in particular is to be discussed
later in the symposium; many varieties contain inhibitors or toxins which put an
upper limit on their use and the value of their protein is limited by its relative
deficiency of methionine (cf. Aykroyd & Doughty, 1964).
It can be argued that the ‘world protein gap’ (United Nations, 1968) does not
exist and that sufficient protein is almost always available if only it is allocated
efficiently between members of the family. Certainly the occurrence of cases of
protein deficiency amongst weanlings is due largely to ill-informed feeding practices,
particularly those adopted when the child is attacked with diarrhoea (Brock, 1961).
T h e trend in recent years has been to revise downwards the estimates of the protein
requirements of populations as a whole (FAO, 1965). Equally it has been emphasized that if supplementary protein is given to people deficient in overall calorie
intake the protein will be metabolized for energy purposes which have priority
(cf. Gounelle, 1969).
I n our own affluent society, with the problem of overnutrition, we tend to think
‘protein good, calories bad’. For the world as a whole we must think ‘protein good,
calories good’. While encouraging plant breeders who are working to raise the protein and essential amino acid content of staple foods we must equally encourage
the development of higher-yielding varieties even if they have the same chemical
composition-their contribution to better nutrition is at least as great. All over the
world, whenever cereal staples have become more abundant and people have been
able to afford to supplement their diet more liberally, they have done so with highprotein, high-vitamin materials. Moreover, any surpluses of staples can be fed to
livestock and converted into foods, such as eggs and milk, ideal for vulnerable
groups. At the same time a moderate surplus in good years provides a margin of
safety for adverse years.
I am indebted to Colborn Vitafeeds Ltd for help with the use of an analogue
computer and matrices for formulating least-cost diets.
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The physiology of the synthesis of amino acids and their movement into
the seed proteins of plants
By B. F. FOLKES,
School of Biologicul Sciences, University of East Angliu, Norwich
As far as is possible I have combined information derived from the work of two
research groups in order to describe an idealized plant. Professor E. W. Yemm’s
group at Bristol, with which 1 was associated for over 20 years, carried out in the
period 1945-60 a fairly extensive investigation of the amino acid metabolism of
barley (Hmdeum distichon Spratt Archer), while more recently D r J. S. Pate and his
colleagues at Belfast have provided us with a most comprehensive description of the
synthesis of amino acids and carbohydrates in the field pea (Pisum arvense). Much
of this work has been reviewed by Yemm & Folkes (1958) and Pate (1968). These
two series of investigations are complementary, both because of the very different
character of the plants involved and because of the use by the Bristol group of
15N as a tracer while the Belfast group used exclusively 14C.
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